This invention relates to the field of digital access arrangement circuitry. More particularly, this invention relates to digital access arrangement circuitry for connecting to a variety of phone line standards. The digital access arrangement circuitry may further include isolation barrier utilizing a capacitor coupled isolation barrier.
Direct Access Arrangement (DAA) circuitry may be used to terminate the telephone connections at a phone line user""s end to provide a communication path for signals to and from the phone lines. DAA circuitry includes the necessary circuitry to terminate the telephone connections at the user""s end and may include, for example, an isolation barrier, DC termination circuitry, AC termination circuitry, ring detection circuitry, and processing circuitry that provides a communication path for signals to and from the phone lines.
Generally, governmental regulations specify the telephone interface requirements and specifications for a variety of parameters including AC termination, DC termination, ringer impedance, ringer threshold, etc. For example, Federal Communications Commission (FCC) Part 68 governs the interface requirements for telephones in the United States. However, the interface requirements world wide are not standardized, and thus, in countries other than the United States the applicable standards may include the TBR21, NET4, JATE, and various country specific PUT specifications. Because the interface requirements are not standardized from country to country, often different DAA circuitry is required for use in each country in order to comply with the appropriate standard. The requirement for different DAA circuitry, however, limits the use of one phone line interface in a variety of countries. Thus, for example, a modem in a laptop computer configured for interfacing with a phone line in one country may not necessarily operate properly in another country. Further, the requirement for different DAA circuitry in various countries hinders the design of a single integrated cost effective DAA solution for use world wide.
As mentioned above, the telephone interface requirements generally include specifications for DC termination of the telephone line. For example, the DC impedance that the DAA circuitry presents to the telephone line (typicallyxe2x89xa6300xcexa9) may be required by regulations to be less than the AC impedance that the DAA circuitry presents to the telephone line (typically≈600xcexa9). Consequently, inductive behavior is required from the section of the DAA circuitry that sinks DC loop current, which is typically called the DC termination or DC holding circuitry. This inductive behavior of the DC holding circuitry should provide both high impedance and low distortion for voiceband signals. The DC termination specifications may also include limits for the maximum current and power dissipation. For example, the TBR-21 specification requires the DC holding circuit to limit DC current to less than 60 mA with a maximum power dissipation of approximately 2 watts.
Prior techniques for implementing DC holding circuitry have included bipolar transistor (e.g., PNP transistor) implementations. These prior techniques, however, have suffered from various disadvantages. For example, although bipolar transistor implementations typically present a desired high impedance (e.g., greater than  greater than 600xcexa9) to the telephone network for voiceband signals, such implementations are limited. In contrast, a CMOS design would be preferable because CMOS technology allows a high level of integration, for example with other phone line interface functions. CMOS implementations on CMOS integrated circuits, however, may face considerable problems in dissipating the power consumed by the DC holding circuitry. The design of a DC holding circuit for use with multiple standards may be further complicated in that the various international specifications may conflict with regards to off-hook settling times and pulse dialing templates (which may require fast settling time constants) and high speed interface designs. (such as for use in modems) which require very low frequency operation (i.e. approximately as low as 10 Hz). Furthermore, it is desirable to implement such DC holding circuits in a manner that does not cause excessive distortion at low and high frequencies.
It is also desirable that the DAA circuitry act as an isolation barrier since an electrical isolation barrier must exist in communication circuitry which connects directly to the standard two-wire public switched telephone network and that is powered through a standard residential wall outlet. Specifically, in order to achieve regulatory compliance with Federal Communications Commission Part 68, which governs electrical connections to the telephone network in order to prevent network harm, an isolation barrier capable of withstanding 1000 volts rms at 60 Hz with no more than 10 milliamps current flow, must exist between circuitry directly connected to the two wire telephone network and circuitry directly connected to the residential wall outlet.
There exists a need for reliable, accurate and inexpensive DAA circuitry for effecting the DC termination characteristics for multiple phone line standards and a DAA circuitry which also provides the necessary electrical isolation barrier.
The above-referenced problems are addressed by the present invention, which provides a reliable, inexpensive, DAA circuit that may be utilized with multiple telephone interface standards and which also provides an isolation system that is substantially immune to noise that affects the timing and/or amplitude of the signal that is transmitted across the isolating element, thus permitting an input signal to be accurately reproduced at the output of the isolation system.
The present invention provides digital direct access arrangement (DAA) circuitry that may be used to terminate the telephone connections at the user""s end to provide a communication path for signals to and from the phone lines. Briefly described, the invention provides a means for providing DC termination for a variety of international phone standards. The invention may also be utilized with means for transmitting and receiving a signal across a capacitive isolation barrier. More particularly, a DC holding circuit is provided that is a second order circuit. The use of a second order circuit provides improved distortion characteristics, particularly at low frequencies.
In one embodiment, a communication system is provided. The system may comprise phone line side circuitry that may be coupled to phone lines and powered side circuitry that may be coupled to the phone line side circuitry through an isolation barrier. The system may further include a DC holding circuit within the phone line side circuitry, the DC holding circuit having at least two frequency poles.
In another embodiment, a method of providing a communication system that may be coupled to a phone line is provided. The method may include coupling an isolation barrier between powered circuitry and phone line side circuitry, and forming a DC holding circuit within the phone line side circuitry, the DC holding circuit comprising a second order circuit.
In yet another embodiment, a DC holding circuit for a communication system that may be connected to phone lines is provided. The DC holding circuit may comprise an integrated circuit, external circuitry external to the integrated circuit, and internal circuitry within the integrated circuit, the external circuitry and the internal circuitry being coupled together. The circuit may further include at least a first capacitor within the external circuitry and at least a second capacitor within the external circuitry, the first and second capacitors providing at least a second order DC holding circuit.
In an alternative embodiment, a DC holding circuit is provided. The circuit may comprise at least one input, the input receiving phone line DC loop current, a first frequency pole generating circuit within the DC holding circuit coupled to the at least one input, and a second frequency pole generating circuit within the DC holding circuit, wherein the DC holding circuit is at least a second order circuit.
In still another embodiment, a method of operating a DC holding circuit is provided. The method may include providing a DC holding circuit, generating a first frequency pole within the DC holding circuit, and generating a second frequency pole within the DC holding circuit such that the DC holding circuit is at least a second order circuit.